1. Field of the Invention
This invention relates to a method for preparing polymers. More particularly, this invention relates to a method for preparing asymmetric radial polymers.
2. Prior Art
Heretofore, several methods have been proposed for preparing asymmetric radial polymers. As is well known in the prior art, radial polymers comprise three or more arms extending outwardly from a nucleus. The asymmetric radial polymers, generally, contain arms of at least two different polymers, which polymers may vary as to chemical composition, structure and/or molecular weight. A principal difference in the method frequently used to prepare asymmetric polymers resides in the selection of a coupling agent which forms the nucleus of the radial polymer. The coupling agent may contain a fixed, though sometimes variable, number of functional sites such as the coupling agents taught in U.S. Pat. Nos. 3,281,383; 3,598,884; 3,639,517; 3,646,161; 3,993,613 and 4,086,298 or the coupling agent may itself be a monomer which polymerizes during the coupling reaction such as taught, for example, in U.S. Pat. No. 3,985,830.
In general, and when an asymmetric polymer is prepared using one of the methods heretofore proposed, a blend of polymeric arms is first prepared containing the various polymeric arms in the desired ratio and the blend of polymeric arms is then added to the coupling agent or the coupling agent is added to the blend of polymeric arms. These methods do, then, result in the production of a product having, on average, the desired number of each kind of arm in the asymmetric polymer. The real problem associated with producing asymmetric polymers in this fashion, however, is that the product obtained is in actuality a statistical distribution of all possible products.
Recently, it has been discovered that a more uniform asymmetric polymer can be produced by combining the arms and a nonpolymerizable coupling agent sequentially. This process takes advantage of the fact that the first arms combined with the coupling agent will react with the next most active sites on the coupling agent and so forth. Such a process is taught in copending U.S. patent application Ser. No. 07/593,378. One disadvantage of this process is that the last arms to couple with the coupling agent; i.e., those arms which couple with the least reactive of the coupling agent sites proceed very slowly and it may take even hours to complete the preparation of the asymmetric polymer. The need, then, for a method to expedite the coupling reaction to completion when preparing a relatively uniform asymmetric polymer is believed to be readily apparent.